1. Field of the Invention
This invention relates generally to powder feed systems and to delivery methods for feeding and depositing finely divided or particulate material into a die cavity of a powder press for compacting. More particularly, the invention relates to a feeding system and delivery method that provides for uniform density distribution of the particles throughout the die cavity. The feeding system and delivery method also provides a uniform, predetermined constant weight of particulate material into the die cavity. In one embodiment, the invention provides an apparatus and method for delivering particulate material to the die cavity of a powder press without the use of a shuttle. The invention is also directed to a feeding system that includes a scale apparatus for accurately measuring the weight of particulate material before it is delivered to a die cavity.
2. Description of the Related Art
In powder metallurgy, products and parts are formed by pressing finely ground or atomized metal powders into a desired shape within a die cavity of a powder press.
Generally, the metal powders are compacted in the die cavity at room temperature and the then semi-dense "green" compact is removed from the die and heated at very high temperatures (at or near the melting point of the material) to bond the powders into a unified mass. The heat bonding procedure is generally known in powder metallurgy as sintering or analogously in the fields of ceramics and carbides, firing.
When these and similar procedures are employed, means are required for delivering amounts of powder or particulate material to the die cavity of the press. Typically, feed shoes operate to deliver the powder or particulate material to the die cavity during the press cycle by using a gravity fill system. This system involves the movement of the feed shoe containing particulate material on a shuttle which slides the shoe forward along the table of the die press to a position at which the bottom feed hole of the feed shoe is exposed, overlies and registers with the die cavity furnishing enough loose powder under the action of gravity to fill the die cavity volumetrically. Thereafter, the shuttle slides the shoe back along the table of the die press into a retracted position. This action cuts off the gravity induced flow of particulate material from the bottom hole of the feed shoe. The particulate material in the die cavity is then compressed into an article and the article is ejected from the die. The shuttle then slides the shoe forward along the table of the die press displacing the ejected article and again exposing the bottom hole of the feed shoe as it overlies and registers with the die cavity. Gravity is thereby once again used to fill the die cavity with particulate material more or less volumetrically. Very small recesses of the die cavity do not however fill uniformly. The feed shoe is then once again retracted to cut off the gravity flow of particulate material into the cavity.
The aforementioned typical example of a feed shoe delivers particulate material by volume (volumetric). Such volumetric feed shoes depend on a consistent bulk density and good flow characteristics (low Hall Numbers) of the powder material they are delivering for an accurate and uniform feed rate. However, because many of the powdered materials used are heavy and dense, they have a tendency to self compact. Furthermore, the die cavities used to make very large parts with fine detail are particularly difficult to fill uniformly. Thus, these and other volumetric feed shoes and delivery methods are generally inadequate and unsatisfactory to provide for very precise uniform distribution and hence density of the powder throughout the die cavity. Consequently, the density of parts made from these powder compacts is not uniform throughout and is not consistently uniform from part to part. These parts are then prone to stress related cracking, especially upon ejection from the die cavity. To make matters worse, the cracks oftentimes become visible only upon sintering.
Additionally, complex part shapes and parts having tight dimensional tolerances such as helical gears and sprockets cannot be satisfactorily produced using commonly available powder feed methods and feed shoes. Since these prior art methods and feed shoes use gravity alone to induce the flow of the particulate material into the die cavity and are thus unable to deliver powder uniformly to all regions of the complex die shapes needed to produce an item such as a helical gear or sprocket.
Specifically, powder simply falls from the feed shoe into the die without any particular pattern or regularity of density. Certain regions of the dies, particularly complexly shaped dies, receive more particulate material than other regions. The resultant parts are therefore irregular in density, subject to failure, and as a result, of questionable commercial use.
Traditional volumetric powder feed methods are further hindered by the inability to control the weight of material delivered into the die making it impossible to provide uniform weight from part to part. Hence, this further limits the uses for parts made by powder metallurgy.
Typically, as a solution to the problem of irregular powder density in the die cavity, when using a powder such as for example, aluminum powder, shaking or vibrating the feed hopper is employed for inducing flow of the particulate material and regularizing the density of the powder in the die. This is, however, time consuming, inaccurate, and not adequate to achieve sufficiently uniform density from part to part and throughout the part itself.
As a further disadvantage, shaking fine powders dislodges "fines" and dust from the powder which are then air borne to coat and contaminate the surrounding environs. Many of the powdered materials used in powder metallurgy to make parts are frequently quite costly and in some cases toxic. Airborne aluminum powders are also quite explosive. Hence, the dust problem can represent a considerable economic loss or health and safety hazard. Consequently, relatively elaborate and costly dust recovery systems and personnel safety precautions, such as filtered masks are presently used.
U.S. Pat. No. 3,697,208 to Munk et al. is directed to an apparatus for filling molds with comminuted fibrous materials by blowing the material into the mold. The air used to blow the material into the mold escapes out of a perforated plate or screen placed on top of the mold for preventing the loss of material from the mold during the blowing step. This apparatus is however, unsuitable for delivering all types of particulate material into a mold, especially metals which tend to be heavier and would therefore not move in the open system described in Munk et al. The Munk et al. process works like a sand-blaster to draw powder into an air stream that precedes the powder supply. The air to powder ratios are large and the time to fill is small. Moreover, uniform density of pressed powder parts could not be satisfactorily obtained because of the powders that fly up toward the perforated plate or screen during delivery. The requirement of a screen also makes it impossible to make parts that do not have flat top surfaces.
U.S. Pat. No. 4,813,818 to Sanzone discloses a feed shoe having a hopper for receiving powder materials from a source that communicates through a feeder tube with an enclosed filling chamber. The filling chamber is equipped with a vacuum. The vacuum is applied to assist the gravity flow of the powders through the feeder tube into the filling chamber. Such evacuation of the chamber does not however provide for the strict uniformity of density that is necessary to produce articles such as materials for thermal management applications, articles having tight dimensional control, etc. In thermal management materials strict uniformity of properties (i.e., coefficient of thermal expansion, thermal conductivity, etc.) throughout the article and from article to article is required. The Sanzone evacuation also does not provide for delivery of precisely controlled weights of powder to the die cavity. Further, using Sanzone, the driving force on the powders can never exceed atmosphere.
In addition, there presently exists technology for controlling and moving mechanical parts of die presses at much faster rates than are presently being used. The rate at which die presses can produce articles is limited by the rate at which the die cavity can be filled with particulate materials. This rate is relatively slow using known methods for delivering particles and feed shoes which use gravity to feed powders into the die cavity. Hence, known methods do not therefore allow the die presses to reach their maximum capacity for producing parts. The rate of production for the die presses is even slower using vibratory methods to induce more regular powder flow.
Moreover, in some instances there is an additional loss of time involved in shuttling the feed shoe back and forth from the die cavity to a retracted position in order to avoid the upper punch of the die press during the press cycle or "stroke." The stroke time is also lengthened by the requirement to allow sufficient time to raise the upper punch high enough to allow the feed shoe to pass thereunder en route to a retracted position.
In the aforementioned and other known methods and feed shoes for delivering particulate material to a die cavity, the step of retracting the feed shoe by dragging the feed shoe over the upper surface of the wear plate of the die table has been necessary to cut off the flow of particulate materials from the feed shoe. This retraction of the feed shoe after filling the die cavity results, however, in the buildup of powders in the die near the trailing edge of the feed shoe. This friction induced "wedging" effect further exacerbates the problem of obtaining parts and articles having non-uniform density upon compacting the particulate material in the die cavity, with all of the accompanying aforementioned disadvantages.